The intake system of a typical internal combustion engine has an air cleaner that includes a trap element for trapping foreign matter such as dust in intake air. In recent years, to suppress the leakage of fuel vapor to the atmosphere through an intake system from the combustion chamber of an internal combustion engine when the engine is not running, air cleaners have been developed that include a filter sheet for adsorbing fuel vapor. The filter sheet is located upstream of a trap element. For example, Japanese Laid-Open Patent Publication No. 2002-276486 discloses such an air cleaner. Specifically, a flexible filter sheet formed by sandwiching granular activated carbon with nonwoven fabric sheets and heat-resistant nets is arranged in a housing, while being laid on a trap element. The filter sheet and the trap element divide the interior of the housing into an inlet chamber and an outlet chamber. An inlet port is connected to the inlet chamber. The inlet chamber is exposed to the atmosphere through the inlet port and an intake duct connected to the inlet port. On the other hand, an outlet port is connected to the outlet chamber. The outlet chamber is connected to the combustion chamber of an internal combustion engine through the outlet port and an intake manifold connected to outlet port.
When the engine is running, air drawn into the inlet chamber from the atmosphere through the intake duct and the inlet port first passes the trap element of the above air cleaner. The trap element traps dust in the intake air. The filtered air flows into the outlet chamber after passing the filter sheet, and is then drawn into the combustion chamber through the outlet port and the intake manifold. On the other hand, when the engine is not running; fuel vapor can flow into the outlet chamber from the combustion chamber through the intake manifold and the outlet port. In such a case, the fuel vapor is adsorbed by the granular activated carbon of the filter sheet, so that the fuel vapor is prevented from leaking into the atmosphere.
As described above, the filter sheet described above is closer to the outlet port than the trap element. In other words, the filter sheet is located downstream from the trap element with respect to the flowing direction of intake air. Thus, when the engine is running, the flow of intake air can displace the filter sheet toward the downstream. Particularly, a part of the filter sheet that is close to the opening of the outlet port and allows passage of intake air is likely to be displaced since great amount of air passes through the part. Such displacement substantially reduces the cross-sectional area of airflow in the vicinity of the outlet port. This may hinder smooth flow of air flowing into the opening of the outlet port, which increases the flow resistance in the intake system.
Also, when passing through the filter sheet, air can cause ruffling of the filter sheet. This causes pulsation of the intake air. Such pulsation of intake air can cause the intake performance of an internal combustion engine to deteriorate.
These drawbacks are not particularly unique to the air cleaner provided in the intake system of an internal combustion engine described above, but are commonly observed in other types of gas filtering devices provided with a flexible filter sheet.